Microwave chamber having energy density control system

ABSTRACT

A microwave treating chamber is disclosed which includes a monitor for determining the average energy density of the microwave field within the chamber and a feedback control system which responds to the monitor by maintaining the energy density within the chamber constant. The monitor includes a duct which extends a substantial distance through the chamber and through which water is passed at a constant rate. Thermistors in the duct on opposite sides of the chamber measure the total thermal energy imparted to the water by the radiation within the chamber. This total energy gain provides a precise indication of the average energy density along the path of the duct of the microwave field. An electrical feedback loop from the monitor is included for controlling the intensity of the radiation fed to the chamber in order to maintain the energy density constant.

United States Patent White 1 May 23, 1972 MICROWAVE CHAMBER HAVINGENERGY DENSITY CONTROL SYSTEM Examiner-J. V. Truhe ABSTRACT A microwavetreating chamber is disclosed which includes a monitor for determiningthe average energy density of the microwave field within the chamberand'a feedback control system which responds to the monitor bymaintaining the energy density within the chamber constant. The monitorincludes a duct which extends a substantial distance through the chamberand through which water is passed at a constant rate. Thermistors in theduct on opposite sides of the chamber measure the total themial energyimparted to the water by the radiation within the chamber, This totalenergy gain provides a precise indication of the average energy densityalong the I path of the duct of the microwave field. An electricalfeedback loop from the monitor is included for controlling the intensityof the radiation fed to the chamber in order to maintain the energydensity constant.

7 Claim, 3 Drawing Figures [72] Inventor: Jerome R. White, San Carlos,Calif.

73 Assignee: Varian Associates, Palo Alto, Calif.

[22] Filed: Feb. 9, 1970 [211 App]. No.: 9,769

[52] U.S.Cl -..2l9/l0.55, 324/95 [51] lnt.Cl. ..H05b 9/06 [58] FieldofSearch ..219/l0.55; 324/95, 106; 73/190, 193, 355

[5 6] References Cited UNITED STATES PATENTS 2,560,536 7/1951 Althouse..324/95 X 2,850,702 9/1958 White ..324/95X 2,866,950 12/1958 Smits..324/95 X 3,281,568 10/1966 Haagensen ..219/10.55 3,365,562 1/1968Jeppson ..219 10.s5

BIAS LINE VOLTAGE Patented May 23, 1972 3,665,140

F I G. I

RESISTANCE T0 CURRENT CONVERTOR 57 "H 56 F |G.2 30 KW. i +8V. g '3 I- Si P'- \4y 5 0 E: BIAS LINE VOLTAGE g v INVENTOR.

0 M I F JEROME R. WHITE ATTORNEY MICROWAVE CHAMBER HAVING ENERGY DENSITYCONTROL SYSTEM BACKGROUND OF THE INVENTION This invention relates toelectromagnetic radiation treating apparatu'sand, more particularly, tosuch anapparatus having means for measuring and controlling the averageenergy density of the radiation in the treating zone.

In electromagnetic radiation treating apparatus, such as microwavecooking equipment, it is often desirable to be able to determine withaccuracy the average energy density of the electromagnetic field in thetreating chamber or zone. Such density provides a measure of the degreeof cooking or heating to which a product in the zone is subjected by theradiation field. One measure of this average density is provided by theaverage of the absolute value or square of the intensity of the field inthe zone. In the past, to determine this density, it has been thepractice to locate one or more microwave probes at selected positionswithin the chamber. However, as is known, the energy density within anelectromagnetic treating chamber or zone is generally not uniform. Thatis, because of the presence of standing waves and because of modemixing, the energy density may vary with both space and non-harmonicallywith time throughout the zone. Because of this, the reading obtainedfrom a microwave probe is only instantaneously representative of theenergy density and only at the one particular location at which theprobe is positioned. The reading obtained from it, therefore, does notrepresent the average energy density throughout the zone.

In attempting to provide a better reading, those skilled in the art haveplaced a plurality of microwave probes at selected locations within thezone and then averaged the various readings. While this procedure willprovide better results than that obtained with one probe, it still doesnot providea truly accurate measure of the average density throughoutthe zone. This is so because the error of any one probe may often onlybe compounded by like errors at other probes. An inordinate number ofprobes would have to be located throughout the 'full zone before astatistical average reading of all of them would provide a usefullyaccurate measurement of the average energy density in the zone. Besidessuch an arrangement being quite expensive, it is impractical.

The inability to obtain anaccurate measure of the average energy densitywithin a microwave treating zone has precluded the commercial use ofmicrowave processing in many instances. For example, in the continuouscooking of food pieces such as chicken parts by passing the same on aconveyor through a cooking chamber, it is often necessary before acooking process will be acceptable from a commercial standpoint that allof the food pieces be subjected to the same amount of cooking to withina degree or two of temperature.

Since those in the microwave cooking art have been incapable of evendetermining the energy density with the accuracy needed, they have notbeen able to provide this required cookingcontrol. This problem has beencompounded by the fact that the energy density of a microwave fieldwithin a chamber depends largely on the amount of absorptive material tobe treated which is present in the chamber at any one time. That is, asmore product is added to the chamber, more energy will be absorbed witha consequent lowering of the energy density in the chamber assuming, ofcourse, that the input power is maintained constant. Thus, the inabilityto obtain an accurate measure of the density so that the power sourcecan be controlled to provide the desired heating rate has prevented theuse of microwave heating or treating in many possible applications.

SUMMARY OF THE lNVENTlON The present invention provides anelectromagnetic radiation treating apparatus and a heating rate monitorfor the same which is quite simple and yet provides the necessaryacwhich is based upon the monitor and which assures that the energydensity within the treatment zone is maintained at a desired levelirrespective of the amount of absorptive product within such zone. Inits basic aspects, the monitor of the invention includes sensing meansresponsive to the electromagnetic radiation within the treatment zone bygenerating a representation, such as an electrical signal or a thermalgain, of the energy density of the field thereof. Such means extends asubstantial distance through the electromagnetic field and senses theenergy density along its path through the field. This provides anintegration of the energy density over the path of the sensing means,and a measurement or sensing of such integration will provide anaccurate indication of the average energy density along such path. Ifthe field along the path of the sensing means is representative of thefield in the full treatment zone, the measurement obtained isrepresentative of the average energy density in such zone. In thoseinstances in which the treating zone is a heating zone, the sensingmeans most desirably provides the desired representation as a thermalgain. Then the sensed representation is proportional to the true heatingrate along the path, as well as to the average energy density. In apreferred embodiment, such means responsive to the electromagneticradiation includes a duct which is transparent to the radiation withinand which passes a substantialdistance through the chamber. Means areprovided for passing at a predetermined rate through the duct a flowablematerial which is heatable by radiation in the zone. Measurement of thegain of thermal energy imparted to the fiowable material by theradiation as it passes through the chamber is a simple matter ofobtaining the desired accurate measurement of the average energy densityof the field along the path of the material. That is, the thermal energygained by the material is directly related to the energy density of theradiation causing the gain and measurement of the total gain in energyimparted to the material as it passes through the zone provides anenergy density measurement which takes into account both time and spacefluctuations in the energy along such path, i.e., the average energydensity.

The invention also provides means which responds to changes in theenergy density by changing the power output of the radiation source tocontrol such energy density, e.g., maintain the same constant. When itis maintained constant, one is assured that all pieces of productexposed to radiation within the zone for a given time will be subjectedto the same radiation energy, irrespective of those factors which willnormally cause field variations.

BRIEF DESCRIPTION OF THE FIGURES With reference to the accompanyingdrawing, FIG. 1 is a schematic illustration of a preferred embodiment ofthe invention;

FIG. 2 is a graphical representation of the transfer characteristics ofa portion of the control system of the invention; and

FIG. 3 is a graphical representation of the transfer characteristics ofthe radiation power source of the preferred embodiment.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT With reference to FIG. 1of the accompanying drawing, a microwave treatment zone is defined by aradiation confining chamber schematically illustrated at l l. Insofar asthe instant invention is concerned, chamber 11 can be designed toconfine or support microwave radiation in any suitable manner. Forexample, the chamber can be either a standing wave resonant cavity or atravelling wave guide.

Means are provided for supporting within the chamber one or more objectsto be treated. For this purpose, opposite end walls 12 and 13 of thechamber are provided with registering feedthrough slots 14 through whichpass a conveyor belt 16. A conventional belt drive (not shown) isprovided to continuously move belt 16 through the slots 14 in thedirection of arrow 17. The belt 16 acts as means for moving a pluralityof objects to be treated, such as chicken parts to be cooked, throughchamber 1 1 in a continuous treating process.

Means are provided for delivering microwave energy to the treatment zonedefined by chamber 11. That is, a microwave generator 18 feeds microwaveenergy through waveguide coupling 19 into the chamber 11. Generator 18can be of any suitable type. in the preferred embodiment beingdescribed, it is of the self-oscillating type disclosed and claimed inUS. Pat. No. 3,461,401, the disclosure of which is hereby incorporatedherein by reference. More particularly, the microwave generator 18includes a klystron amplifier 21 which has a portion of its power outputfed back to its input via a coupler 22 and an electrically longtransmission line feedback path, schematically represented at 23, havinga length equalto or greater than Q wavelengths long where Q is theinverse of the fractional bandwidth of the amplifier and feedback pathover which the loop gain is greater than unity. An electronicallyvariable attenuator in the form of a PIN diode modulator 24 is includedin the feedback path to permit regulation of the power level of theoscillator for a purpose to be described subsequently.

As has been mentioned previously, the energy density of the microwaveradiation fed to chamber 11 by generator 18 will not be uniformthroughout the chamber. That is, due to mode mixing and the presence ofstanding waves within the chamber, the uniformity of the fieldthroughout the chamber will vary both spatially and with time. This hasmade it difficult to obtain an accurate reading of the average energydensity within the chamber. Such a reading is necessary in order toprovide a measure of the amount of radiation treatment to which theobjects being treated are exposed, and also to permit control of suchamount. The problem becomes especially acute when it is desired to treatmaterials 'on a continuous process, i.e., when it is desired tocontinuously pass a plurality of objects through the chamber at apredetermined rate to treat the same equally with microwave energy. Thisis because the energy density of the microwave fieldwill vary dependingupon the amount of product within the chamber. That is, as more productenters the chamber, more of the energy will be absorbed, thus loweringthe energy density. However, to assure that all of the' product receivesthe same heating or amount of cooking, the density should be maintainedconstant. Before such can be done, though, an accurate measurement ofthe average energy density throughout the chamber must be obtainable. v

The instant invention provides a heating rate monitoring arrangementcapable of obtaining the desired accurate measurement. To this end, aduct in the form of a tube 26 extends through chamber 11 between endwalls 12 and 13. (Chamber 11 is shown cut away to better illustrate thelocation of such tube). Means are provided for passing at apredetermined rate through the tube a flowable material which will beheated by radiation in the chamber. Most desirably, the flowablematerial is a liquid such as water which is passed through the duct at aconstant rate for simplicity. Such means is graphically represented as apump 27 which forces the flowable material through the tube 26 in thedirection of arrows 28. Means are provided for measuring the gain inthermal energy imparted to the material in its passage through thechamber. For this purpose, a pair of temperature-to-signal typetransducers in the form of resistance bulb thermometers or thermistors31 and 32 are provided in tube 26 respectively upstream and downstreamof chamber 11. Thermistor 31 measures the temperature of the water orother flowable material prior to its entry into chamber 11, andthermistor 32 provides its temperature after it has passed through suchchamber. The difference between the entrance and exit temperatures ofthe liquid will provide a precise indication of the total gain inthermal energy imparted to the flowable material by the radiation withinthe chamber. This total gain in thermal energy will be directly relatedto the average energy density of the field along the path of theflowable material during itstransit time through the chamber. That is,fluctuations in the energy density along such path will causecorresponding fluctuations in the thermal energy imparted to theflowable material.

The length of the path or distance within the chamber that must betraversed by the duct before the thermal energy gained by the materialflowing therethrough will be representative of the average energydensity throughout the chamber, rather than just along such path, willdepend upon the particular treatment'chamber and radiation frequencybeing used. As will be apparent, this distance should be at least aslong as several wavelengths of the radiation and for most applicationsat least five wavelengths long before the density measurement will berepresentative. Whenever it is stated herein and in the claims that thedensity measuring means or duct extends a substantial" distance throughthe treatment zone, it is meant that the duct extends a sufficientdistance through the chamber to provide an average energy density of theaccuracy desired for the particular application.

As is illustrated in FIG. 1, the duct 26 through the chamber ingenerally the same direction as the objects or product being treated aremoved therethrough. It will be appreciated that with this arrangementthe duct is subjected to substantially the same fluctuations in energydensity that the product being treated is subjected to. Thus, theaverage energy density or heating rate provided by the thermal gain ofthe flowable material is closelyallied to the energy density or heatingrate to which the product is subjected, irrespective of whether or notthe thermal gain provides an accurate indication of the average energydensity throughout the full chamber. For best results, the duct shouldbe spaced from the walls of the chamber in order to preventperturbations, etc. at such walls from affecting the measurement. Also,as is illustrated, the

duct 26 is surrounded within the treatment chamber 11 with a closed heatinsulation jacket 30. Jacket 30 is transparent to the radiation withinthe chamber, and the dead air space between it and duct 26 prevents anythermal energy within the chamber, other than that imparted to the duct26 and fluid flowing therethrough by electromagnetic radiation, frombeing sensed by the fluid and affecting the measurement provided by it.The use of such an insulation means can be quite important if theproduct is being treated within the chamber with a heating medium suchas steam in addition to the electromagnetic radiation. The jacket 30 andthe dead air space will prevent thermal energy from the additionalheating medium from reaching duct 26 while not affecting the thermalenergy imparted to it by the radiation since the jacket is transparentto such radiation.

In many instances, it is desirable to be able to control the energydensity of the radiation within a treatment zone. For

example, in the microwave cooking of food pieces in a continuous processby passing the same through-the'chamber, it is necessary that each ofthe pieces be subjected to the same amount of cooking. However, theenergy density and thus cooling rate within a chamber will varydepending on many factors, including the number of food pieces which arein the chamber at one time. The instant invention includes a controlfeedback loop, generally referred to by the reference numeral 33, whichis responsive to a change in the average energy density or heating rate,i.e., in the amount of gain in thermal energy of the flowable material,by causing a corresponding inverse change to the intensity of theradiation delivered to the chamber so that the average energy density oftheradiation field within the chamber is maintained substantiallyconstant. More particularly, the output from the thermistors 31 and 32is fed to a resistance-to-current converter 34. Converter 34 can be ofthe self-balancing bridge type such as that marketed bythe Foxboro Co.,Foxboro, Massachusetts, under Model 694A. Converter 34 is conventionallypowered by a power source 36 which supplies volt alternating current.Such power source 36 is connected to the converter 34 through acontactor or relay 37, the purpose of which will be describedhereinafter. I

The output of resistance-to-current converter 34 is inversely related tothe temperature differential between thermistors 31 and 32 as isillustrated by the converter transfer characteris tics shown in FIG. 2.Such output is used to control the bias on PIN diode modulator 24 and,hence, the output of klystron 21. More particularly, modulator 24 isnormally biased ofi by the application of a negative potential such asone volt to terminal 38 on the end of bias line 39. The output ofconverter 34 is connected through a time lag circuit in the form of lowpass filter 41 to line 39 in order to selectively overcome such bias. Asis shown in FIG. 2, converter 34 has a high'positive output current whenthere is no temperature difference between thermistors 31 and 32, i.e.,when there is little or no microwave intensity within the chamber, inorder to turn modulator 24 and hence klystron 21 full on. The outputvoltage of converter 34 decreases as the temperature difference betweenthermistor 31 and 32 increases, thus reducing the power on klystron 21.FIG. 3 illustrates the transfer characteristics of a suitable klystronamplifier,such as Model No. PPS-30A available from Varian Associates,Palo Alto, California, having a power output variable between and 30kilowatts.

I The contactor 37 in the power circuit of converter 34 is connectedwith the on-off switch of the power source 18 in a manner assuring thatthe converter is tended inoperable whenever the power source is off,i.e., not delivering energy to the chamber. This will assure that whenthe power source 18 is initially turned on that the converter 34 doesnot immediately cause the source to go to full power and possiblyproduce initial overvoltages in the chamber due to the lack oftemperature differential between the input and output thermistors 31 and32.'Time lag circuit 41 further assures the stable operation of thecircuit by delaying the response of the power source to changes in thethermal energy'to a time which is long, compared to the time in which ittakes the flowable material topass between the thermistors 31 and 32.The control changes fed to the power source will therefore berepresentative of the actual conditions within the chamber at the timesuch changes are made.

EXAMPLE An embodiment of the invention has been incorporated into amicrowave resonant cavity designed to uniformly cook up to three-fourthston of chicken pieces in 1 hour by continuously passing the same throughthe cavity. The cavity has dimensions of 4 ft. by ft. by 38 ft. with thechicken being passed through the chamber along the 38 ft. dimension. Thechamber is powered by two PPS-30A microwave amplifiers available fromVarian Associates, Palo Alto, California which together provide 0-60 Kwof power at 2,450 MHz. The cavity is fed by the power packs to set upstanding wave resonance in all dimensions of the cavity.

The duct for the flowable material is extended 5 feet through thechamber along the 38 ft. dimension. The flowable material is water andis fed through the duct at a constant rate of 1 gallon per minute. Inorder to assure that each piece of chicken received the same amount ofcooking, the average field intensity throughout the chamber had to bevery precisely controlled. Therefore, the resistance to currentconverter was adjusted to provide a full 8V power differential for adifference in temperature of only 1 between the inlet and outletthermistors, as is indicated in FIG. 2. The converter 34 also includedmeans for adjusting the field intensity at which it tended to maintainthe cavity 11. This is represented by the dotted lines on each side ofthe characteristic plot line in FIG. 2. The negative bias applied toterminal 38 was minus 1 volt DC, and the low pass filter was designed toovercome this by converting the 8V output of converter 34 to about a 0.6volt output with approximately a 100 second time lag. More particularly,resistor 42 had a value of 2,200 ohms, resistor 43 a value of 500 ohms,and capacitor 44 a value of 0.25 farads. PIN diode modulator 24 was amodel 8732A PIN diode marketed by Hewlett-Packard, Inc. of Palo Alto,California.

With this arrangement, the average microwave fieldintensity was held towithin i 1.5 percent, resulting in each piece of the chicken beingcooked to the same extent as the other pieces, irrespective of thenumber of pieces within, the chamber. The instant invention thereforemakes the chickencooking process usable for cooking chicken at anydesired rate up to three-fourths ton per hour.

While the invention has been described with respect to the continuousprocessing of individual pieces of food or the like, it will beappreciated that it is useful in batch processing, and in general, anysituation in which it is desirable to obtain an accurate determinationof the average energy density within a radiation-treatment zone or tocontrol such density. And while a preferred feedback control arrangementhas been described, from the broad viewpoint any standard feedback orcontrol arrangement, such as one which controls the on-ofi state of thepower source rather than continuously varies its power output, is usablewith the invention. The duct could also be incorporated into aself-balancing bridge or an arm thereof to provide the desiredindication of changes in energy density. It is therefore intended thatthe scope of the invention be limited only by the terms of the claimsand equivalents thereof.

I claim:

1. Electromagnetic radiation treating apparatus comprising a treatingchamber capable of confining an electromagnetic radiation field, meansfor supporting within said chamber at least one object to be treatedwith electromagnetic radiation, said object extending over a distance inone direction in said chamber, means for delivering electromagneticradiation to the object within said chamber to generate anelectromagnetic field in the vicinity of said object, means including aduct transparent to said electromagnetic radiation extending within saidchamber and through said field along said one direction a substantialdistance at least equal to several wavelengths of the electromagneticradiation, meansfor flowing a heatable material in said duct at acontrolled rate of flow over said distance, the gain in thermal energyof said material along said substantial distance of duct being a measureof the average energy density of said field along the path of said duct,temperature insensitive means for measuring said gain in thermal energyalong said duct, and means responsive to said measured gain in thermalenergy for controlling the intensity of the electromagnetic radiationdelivered to said object to maintain the energy density of saidradiation along said path substantially constant.

2. The electromagnetic treating apparatus of claim 1 wherein said meansfor supporting within said chamber at least one object to be treatedwith electromagnetic radiation includes means for moving said objectthrough said chamber in said one direction.

3. The electromagnetic treating apparatus according to claim 1 whereinsaid means responsive to said measured gain in thermal energy of saidflowable material is a feedback control loop connecting said means formeasuring the gain in thermal energy imparted to said flowable materialwith said means for delivering electromagnetic radiation to said object.

4. The electromagnetic treating apparatus of claim 3 wherein saidfeedback control loop includes an electrical time lag circuit whichdelays the response of the means for delivering the electromagneticradiation to changes in said thermal energy for a time which is longcomparable to the time of passage of said flowable material through theportion of said duct within said field.

5. The electromagnetic treating apparatus of claim 3 further includingmeans for rendering said feedback control loop inoperable whenever saidmeans for delivering electromagnetic radiation to said energy.

6. The electromagnetic treating apparatus of claim 1 wherein said meansfor delivering electromagnetic radiation to said chamber deliversmicrowave radiation thereto, and thermal insulation transparent to saidmicrowave radiation is provided around said duct within said chamber.

7. The electromagnetic treating apparatus of claim 2 wherein said ductextends substantially from end to end of said treatment chamber.

1. Electromagnetic radiation treating apparatus comprising a treating chamber capable of confining an electromagnetic radiation field, means foR supporting within said chamber at least one object to be treated with electromagnetic radiation, said object extending over a distance in one direction in said chamber, means for delivering electromagnetic radiation to the object within said chamber to generate an electromagnetic field in the vicinity of said object, means including a duct transparent to said electromagnetic radiation extending within said chamber and through said field along said one direction a substantial distance at least equal to several wavelengths of the electromagnetic radiation, means for flowing a heatable material in said duct at a controlled rate of flow over said distance, the gain in thermal energy of said material along said substantial distance of duct being a measure of the average energy density of said field along the path of said duct, temperature insensitive means for measuring said gain in thermal energy along said duct, and means responsive to said measured gain in thermal energy for controlling the intensity of the electromagnetic radiation delivered to said object to maintain the energy density of said radiation along said path substantially constant.
 2. The electromagnetic treating apparatus of claim 1 wherein said means for supporting within said chamber at least one object to be treated with electromagnetic radiation includes means for moving said object through said chamber in said one direction.
 3. The electromagnetic treating apparatus according to claim 1 wherein said means responsive to said measured gain in thermal energy of said flowable material is a feedback control loop connecting said means for measuring the gain in thermal energy imparted to said flowable material with said means for delivering electromagnetic radiation to said object.
 4. The electromagnetic treating apparatus of claim 3 wherein said feedback control loop includes an electrical time lag circuit which delays the response of the means for delivering the electromagnetic radiation to changes in said thermal energy for a time which is long comparable to the time of passage of said flowable material through the portion of said duct within said field.
 5. The electromagnetic treating apparatus of claim 3 further including means for rendering said feedback control loop inoperable whenever said means for delivering electromagnetic radiation to said energy.
 6. The electromagnetic treating apparatus of claim 1 wherein said means for delivering electromagnetic radiation to said chamber delivers microwave radiation thereto, and thermal insulation transparent to said microwave radiation is provided around said duct within said chamber.
 7. The electromagnetic treating apparatus of claim 2 wherein said duct extends substantially from end to end of said treatment chamber. 